Geopolymers are inorganic aluminosilicate polymers, which are obtained by polycondensation at high pH values and low temperatures (room temperature). By means of an alkaline medium, Si(OH)4 and [Al(OH)4] monomers or also oligomers initially are released from the solid material. Subsequently, solidification is effected by polycondensation, whereby an aluminosilicate polymer network is formed. This network consists of SiO4 and AlO4 tetrahedrons, which each are linked with other tetrahedrons via four corners.
Geopolymers can be used as binder in the construction material industry, in order to decrease the cement content or ensure faster hardening. Further advantages of geopolymers include the chemical resistance, temperature resistance, high final strength, high density and low permeability.
A multitude of solids such as metakaolin or also fly ash can be used as educts. The use of geopolymers as an alternative binder for concrete opens up the possibility of a new construction material. Its potential chiefly consists in the fact that during the production of classical cements, such as Portland cement, major amounts of the greenhouse gas carbon dioxide are released by the reaction:CaCO3→CaO+CO2.
Geopolymers thus represent an alternative binder, which decrease the CO2 emission and counteract the greenhouse effect.
Since the properties of geopolymers depend on their composition, different geopolymers are produced in practice depending on the requirements profile.
U.S. Pat. No. 4,472,199 for example describes a geopolymer from the silicoalumina family with the following composition: Potassium oxide to silicon oxide 0.3 to 0.38, silicon oxide to alumina 4.0 to 4.2, water to alumina 14 to 20, and potassium oxide to alumina 1.3 to 1.52. The geopolymer thus obtained shows a distinctly pronounced structure, which has ion exchange properties and accordingly can be used in a similar way as zeolites or molecular sieves.
From U.S. Pat. No. 4,509,985 however a polymer is known, which has the following compositions: M2O to silica 0.2 to 0.36, silica to alumina 3 to 4.12, water to M2O 12 to 20, and M2O to alumina 0.6 to 1.35, wherein the letter M can be representative for a member of the group including sodium oxide, potassium oxide or a mixture of sodium oxide and potassium oxide. The solid material thus produced is characterized by a particular early high strength.
While in these two documents the polymer is produced from a mixture of different silicates by adding an alkaline activator and water while stirring continuously and at a slightly elevated temperature, DE 691 05 958 T2 describes a process for producing an aluminosilicate geopolymer in which silicon dusts are used. These silicon dusts are obtained by condensation of silicon oxide vapors from the electrofusion at very high temperatures and have an amorphous structure.
All documents have in common that minerals with a defined composition are used as educts.